Among nonimpact printers, which are rapidly expanding their market share, the one that is the simplest in principle and most suitable for color printing is the ink jet printer.
Regarding this type of printer, it can be said that the so-called drop-on-demand type that ejects ink droplets only when forming dots is predominant. Many types of drop-on-demand piezoelectric ink jet heads that use piezoelectric elements have been disclosed and are represented by the Kyser type disclosed, for example, in Japanese Examined Patent Publication No. 53-12138, the layered piezoelectric actuator type disclosed, for example, in Japanese Unexamined Patent Publication No. 6-4327, and the shear mode type disclosed, for example, in Japanese Unexamined Patent Publication No. 63-252750.
In such a piezoelectric ink jet head, a piezoelectric element deformable by the application of a pulse waveform is mounted at least on a portion of the wall surface of an ink chamber which communicates at one end with a nozzle and at the other end with an ink reservoir, and ink is ejected by deforming this piezoelectric element.
The piezoelectric ink jet head is usually driven in the following way. First, a pulse waveform is applied to the piezoelectric element to deform the portion of the wall surface of the ink chamber in such a manner as to cause the internal volume of the ink chamber to increase and thereby draw ink into the ink chamber. Next, by either removing the voltage from the piezoelectric element or applying a pulse waveform opposite in polarity to the first applied pulse waveform, the portion of the wall surface of the ink chamber is deformed in a direction opposite from the direction in which it was first deformed. This causes the internal volume of the ink chamber to decrease, and an ink droplet is ejected. This is the so-called draw-and-fire driving method.
With the above method, however, oscillations will remain on the ink meniscus after the ink droplet is ejected. The residual oscillations consist of pressure wave oscillations due to the mechanical structure of the ink chamber itself and hydrodynamic surface tension oscillations of the ink itself.
If the next ink eject action is attempted while these oscillations remain, the meniscus oscillations caused by its ink ejection driving are superimposed on the residual oscillations of the meniscus caused by the previous ejection. This causes a difference in meniscus position between the current ink ejection and the previous ink ejection, as a result of which the size and ejection speed of the ink droplet vary and stable ink ejection cannot be accomplished.
The higher the ink ejection speed, the more accurately can the ink dot be deposited. Further, if the voltage applied for ink ejection is the same, the shorter the ink ejection time, the higher the ink ejection speed.
The residual oscillations of the meniscus can be suppressed by setting the length of the ink ejection time equal to the cycle of the pressure wave oscillations occurring due to pressure changes in the ink chamber.
However, if the length of the ink ejection time is set equal to the cycle of the pressure wave oscillations to suppress the residual oscillations, this in turn imposes a restriction on the ejection speed. On the other hand, if a high ink ejection speed is to be obtained while suppressing the residual oscillations occurring after ink ejection, the natural frequency of oscillation of the ink chamber itself must be increased. Since this demands a change in head size, freedom in head design is limited.